1. Field of the Invention
The field of the invention is the ultra high-speed, non-destructive measurement of the mechanical, thermal or optical properties of a material. Ultra High-speed measurement is understood to mean a measurement with a temporal resolution of the order of a picosecond.
2. Description of the Related Art
It is known to obtain measurements with a temporal resolution of this type via optical sampling techniques using two pulse trains respectively designated “pump” and “probe” trains, with pulse period T, each pulse having a duration τ of approximately 100 femtoseconds.
The pump beam causes a disturbance in the material or sample which in response produces an optical signal dependent on the optical properties thereof (reflectivity, absorption, expansion, contraction etc.). The probe beam is delayed by a quantity Tps referred to as the “pump-probe” delay and reads the reaction of the material; it is generally of a low intensity relative to that of the pump beam. The temporal reaction of the material is reconstructed by varying this delay between zero and a duration equal at most to the period T of the pulse train. In practice, this duration is much less than T. Typically T is of the order of 13 ns and the variation in the delay is then typically limited to 2 or 3 ns.
Usually, both the pump and the probe pulse train have the same pulse repetition period T and the sampling is thus termed “homodyne”. FIG. 1 shows, in the case of homodyne sampling, pump and probe pulse trains offset by a delay Tps along with the value of the sample reaction obtained for this delay. An example of an assembly allowing this technique to be made use of is schematically shown in FIG. 2. The homodyne sampling device 100 comprises a laser source 10 connected to a splitter 9 which can split the laser beam into a pump beam, modulated by a modulator 8, and a second, probe beam which is delayed by an optical delay line 11. The purpose of the modulator is to convert the signal to have a higher frequency in order to disengage said signal from the noise in the range of 1/f, f being the frequency of the signal. The two beams are subsequently combined by a combiner 20 before being focussed to appoint onto the sample 200 via a microscope lens 30. In the example in the figure, the sample reaction is obtained by reflection. The reaction is directed to a photodetector 50 after being filtered by a pump beam filter 60. The photodetector is connected to an acquisition system 70, via a demodulator 12 which allows the signal to be re-established in the baseband.
The delay Tps is produced and controlled by an optical delay line comprising a mechanical translation system of a mirror disposed in the path of one of the beams. The delay is related to the translation by the formula:Tps=d/c where d is the length of the delay line and c is the speed of light.
Bearing in mind the orders of magnitude, a length d of 30 μm causes a delay of 100 fs. In practice, the length of the delay line is limited. In fact, a displacement of more than 30 cm significantly affects the focussing to a point of one beam relative to the other. A delay of 10 ns, which requires a length d of 3 m, is thus very difficult to achieve. A limit of 2 to 3 ns on the temporal delay corresponds to this limit on the displacement: the temporal reaction of the material is reconstructed only over approximately 2 to 3 ns.
Moreover, these displacements are achieved with a negative effect on the stability of the focussing to a point of the laser beam onto the sample.
Further, the vibrations brought about by the displacement of the delay line deteriorate the signal to noise ratio and considerably increase the measurement time. It usually takes 30 to 40 minutes to obtain a signal over several nanoseconds, i.e. to effect the different displacements so as to scan the reaction of the material over times of up to Tp or approximately 13 ns.